I wonder, if there's any special way to check that, but as far as I can see – every plug is connected properly.
That was a good advice – even though it didn't kill the noise, but I manged it to get bearable for me playing on my own.
Post D/A Level is set to -32dB – that's the max I can get – of course, I can set the level deeper than that, but at everything -32dB and "less", the noise comes back in full volume.
So I pushed up the input to get fully lid meters and pulled the Post D/A down – that way, I can achieve an acceptable volume with less noise.
Nevertheless, the noise is still there.